HDMI is a specification that integrates audio and video information into a single digital interface to be used with, for example, digital video disc (DVD) players, digital television (DTV), high definition TV (HDTV), set-top boxes, and other audio and/or video devices. An HDMI interface cable is configured to carry multi-channel audio data, as well as standard and high definition consumer electronics video formats. Content protection technology is also available and an HDMI cable may be configured to carry control and status information in both directions.
Transition minimized differential signaling (TMDS) is a signaling technique used in HDMI to produce a transition-controlled, DC-balanced series of symbols from the audio and video inputs. TMDS is more properly a way of encoding the audio and video digital information to protect it from degrading as it travels along the length of the HDMI cable, creating data channels associated with the three original colors (RGB)/intensity aberration (YPbPr), defined as TMDS channels 0, 1 and 2, as well as a video pixel clock, defined as TMDS CLK. In a long string of logic zeros and logic ones, bits are selectively manipulated in TMDS in order to keep the DC voltage level of the overall signal centered around a particular signal threshold. Another signal threshold at the receiver is then used to determine whether a received data bit is at a voltage level equivalent to a logic zero (0) or at a level equivalent to a logic one (1).
FIG. 1 illustrates a conventional prior art implementation of an HDMI system, showing an HDMI source 1 and an HDMI sink 2, coupled together via an HDMI cable 4. Within HDMI source 1, the associated video and audio input signals are applied to an HDMI transmitter 3, which transforms these signals into a set of three signals along TMDS channels 0, 1 and 2, and a separate TMDS CLK signal. The TMDS CLK signal will be used by HDMI sink 2 as a frequency reference for data recovery on the three audio/video channels. In the particular arrangement shown in FIG. 1, this set of four signals are transmitted as simplex signals (i.e., “one-way” transmissions) along copper wires within a standard HDMI cable 4 and transmitted to HDMI sink 2. Other copper wires within HDMI cable 4 are used to support half-duplex transmission (i.e., “two-way” transmission at different times for each direction) of various other signals used in HDMI signaling (DDC, CEC, VDD, GND, etc.). The simplex transmission wires supporting the TMDS signals are thereafter coupled, as shown in FIG. 1, into an HDMI receiver 5 that functions to re-convert the TMDS signals into the proper video and audio signals (as well as the recovered clock signal).
In many conventional transmission/reception systems using HDMI, cable 4 comprises a purely electrical cable (usually composed of copper wires). However, there are major drawbacks associated with the use of an electrical cable as a result of its susceptibility to electromagnetic interference, mainly due to the limited bandwidth of the copper wire. Furthermore, the signals transmitted along copper wire are susceptible to power loss, thus limiting the length of copper-based electrical HDMI cables to less than about 15 meters.
It has been previously suggested to replace copper-based HDMI cables with optical fiber-based HDMI cables. By virtue of using an optical cable, the data rate and cable span may both be significantly increased while avoiding the problems of copper wire. Specific arrangements of these optical fiber-based alternatives are disclosed in U.S. Published Application No. 2007/0233906, published on Oct. 4, 2007. To date, however, these solutions pose their own limitations in supporting the transmission of the multiple TMDS channels. One solution is to use separate fibers for each channel, another solution is to use separate wavelengths for each channel. Either of these solutions introduces additional components and/or sources of loss into the system, increasing its complexity and cost. Another proposed solution is to “serialize” the set of TMDS channels prior to transmission over a single fiber, and then “deserialize” the data at a receiver unit (a technique referred to in the art as SERDES). The proposed SERDES solution, however, creates other problems in terms of requiring the HMDI connection to operate at three times the data rate of the individual channels (for example, at 10.2 Gbps as compared to 3.4 Gbps) and as a result requires the use of expensive, high bandwidth components that may consume more power than their lower speed counterparts.
Thus, a need remains for a optical fiber-based HDMI cable that addresses the concerns of the multiple fiber/multiple wavelength arrangements without requiring the use of more complex components associated with SERDES operations.